In the 1990s he developed cancer and underwent a liver transplant - events which permanently affected his health. He suffered a brain haemorrhage early in July 2010 and died on 22 July 2010 in a St. John's hospital at the age of 46.[1]

The first of these books, The IRA and Its Enemies, Violence and Community in Cork, 1916–1923 (1998), is a study of the organisation's social composition and actions in County Cork during the War of Independence. This book won several awards, including the Christopher Ewart-Biggs Memorial Prize (1998). It attracted significant criticism, "le[ading] to almost as much conflict as the events" described.[3][4][5]

Hart edited British Intelligence in Ireland 1920–21: the Final Reports (2002) a re-print of official British reports released to the British Public Records Office that detailed British military and intelligence postmortem critiques of policy during the Irish rebellion from 1919-1921; and The I.R.A. at War 1916–1923 (Oxford University Press, 2003), a collection of essays on various social, political and military aspects of the IRA in these years. The latter work represents, Hart wrote in the preface, "sixteen years' work on the history of the Irish revolution." Hart's last work was a biography of Michael Collins, Mick: the real Michael Collins (Macmillan, 2006).

Hart contributed to the volume The Irish Revolution (2002), a collection of articles by various historians of the period.[6]

Times Higher Education suggested in 2008 that Hart's work "offers a revisionist version of events that proved highly controversial".[3] Hart denied he was a "revisionist", calling it "pejorative labelling".[7] In his review of The IRA and its Enemies: Violence and Community in Cork, 1916–1923, John M. Regan wrote:

"Hart is neither a statist nor a southern nationalist, though the influence of both ideologies can be traced though his work. His research on localised and specialised topics subverts orthodoxy, but it is his willingness to embrace it when dealing with general explanations which surprises. His exploration of the plight of Protestants in the Free State illuminates the sectarian underbelly of the revolution that a nationalist historiography prefers to ignore. In escalating violence in Cork, Tipperary, or Dublin could Michael Collins, Harry Boland, or Ernie O'Malley be held accountable for raising sectarian tensions in Antrim, Down or Belfast? Was the cost of a southern state the institutionalisation of ethno-religious tensions in a compressed and reactionary northern state? Could revolutionary violence in 1922 and 1968 conceivably be part of one grotesque, protracted process? To accept this argument would, however, be to shatter nationalist icons important to a southern nationalist identity still rooted in its own glorious revolution."[8]

Some of Hart's published claims attracted criticism from other historians and writers,[3][9] including two incidents in The IRA and its Enemies. One was the Kilmichael Ambush of 28 November 1920. Hart challenged the account of commander Tom Barry who stated the Auxiliaries engaged in a false surrender that caused two IRA fatalities, after which Barry refused further surrender calls and ordered a fight to the finish without prisoners. Hart posited this never happened and alleged that Barry ordered the killing of all prisoners.[10]

Hart claimed he personally interviewed two anonymous ambush veterans in 1988-89 and listened to recorded interviews with three further unnamed Kilmichael veterans. The recordings (known as 'the Chisholm tapes') were made in 1970 by Father John Chisholm as research for Liam Deasy's Toward Ireland Free (1973).[11]Meda Ryan, author of Tom Barry, IRA Freedom Fighter (2003), questioned Hart's claim to have interviewed two Kilmichael veterans in 1988 and 1989, claiming only one, Edward "Ned" Young, was still alive from 1987-89. Ryan reported him too ill to have contributed to Hart's research. This assertion was supported in an affidavit published in 2008 by Ned Young's son, John.[12] Ned Young died aged 97 on 13 November 1989. According to Ryan (and 1980s newspaper accounts)[13] The second last surviving Kilmichael veteran, Jack O'Sullivan, died in December 1986. However, Hart dated an additional interview with his second anonymous Kilmichael veteran on 19 November 1989, six days after Ned Young died. Hart claimed his interviewee was an unarmed ambush scout, although the last ambush scout, Dan O'Driscoll, reportedly died in 1967. The last dispatch scout, Seán Falvey, died in 1971.[14] Hart's earlier 1992 PhD thesis, on which his book is based, did not describe this 19 November 1989 interviewee as an unarmed scout. In his thesis, Hart described touring around the Kilmichael ambush site with this interviewee, a claim withdrawn from the book.[15][16]

Niall Meehan, Head of the Journalism and Media Faculty in Griffith College, Dublin, questioned Hart's claims with regard to the "Chisholm tapes", in a review of David Fitzpatrick (ed.), Terror in Ireland 1916-1923 (2012). A chapter on the Kilmichael ambush by Eve Morrison was based partially on access to the tapes. She reported two, not three, as Hart stated, Kilmichael veterans recorded in 1970 by Chisholm speaking on the ambush. One of these two was Ned Young. The other recorded interviewee, Jack O'Sullivan, spoke words which were misattributed by Hart to the ambush scout he claimed he interviewed on 19 November 1989. Meehan asserted that "this misattribution... further questions the existence of Hart’s 1988 and 1989 veteran interviews".[17][18]

The second controversy surrounds the Dunmanway killings, in which thirteen Protestant men and boys were shot dead between 27–29 April 1922 during the truce. Hart wrote, "these men were shot because they were Protestant".[19] Others point to evidence suggesting that, while the IRA action was unauthorised, the men were targeted due to allegations they were informers, not because of their religion. Again, criticism centered on Hart's use of evidence. In his review of The IRA and its Enemies (The Month, September–October 1998) Rev. Brian Murphy (OSB) noted Hart's citation of a British intelligence assessment in the Record of the Rebellion in Ireland that "in the south the Protestants and those who supported the Government rarely gave much information because, except by chance, they had not got it to give." Rev. Murphy pointed out that Hart had omitted the following sentence:

An exception to this rule was in the Bandon area where there were many Protestant farmers who gave information. Although the Intelligence Officer of the area was exceptionally experienced and although the troops were most active it proved almost impossible to protect those brave men, many of whom were murdered while almost all the remainder suffered grave material loss.[20]

As the April killings took place in "the Bandon area", Rev. Murphy queried apparent suppression of evidence contradicting Hart's conclusion. This has been echoed in further discussion.[21][22][23][24][25][26][27][28]

Hart stood by his work, stating that critics have failed to "engage with the book's larger arguments about the nature of the IRA and the Irish Revolution"[3] and believing they are closed to "a real debate where people concede some things and put forward others or are skeptical about weak points and accept the strong points."[9] Hart's last known interview was in a TG4 Irish language programme on Tom Barry, broadcast in January 2011. The programme had access to eight of nine Chisholm tapes (in total) and questioned Hart's use of anonymous sources and other claims.[29]

In his 2011 book, Michael Collins and the Anglo-Irish War: Britain's Failed Counterinsurgency, author J.B.E. Hittle, a retired U.S. career intelligence officer-turned historian, acknowledged Hart's overall contribution in re-examining standard histories of the period, but concluded that Hart's historical method is "problematic". Hittle cited Hart's "overall naivete" about guerrilla warfare, in particular, what he viewed as Hart's underestimation of the importance of certain counterintelligence cases to the outcome of the war, as well as faulty methodologies.[30]